The invention relates to an electrical shutter for a camera, and more particularly, to an electrical shutter of electromagnetic release type in which shutter blades are operated for opening and closing by the resilience of a prime mover spring which is disposed on a single shaft.
An electrical shutter of electromagnetic release type is already known in which a prime mover spring disposed on a single shaft is charged in response to a film winding operation and is released in response to an electromagnetic release to drive a shutter drive member for rotation in order to operate shutter blades and wherein the rotation of the drive member is controlled by a detent member which is constrained by an electromagnet or the like in order to determine the length of an exposure period. Such electrical shutter is simple in construction and provides an automatic stop of the film winding for each picture frame. Additionally, it is constructed so as to prevent a double exposure.
A shutter mechanism of the single shaft rotating type is typically constructed in the manner as shown in FIG. 1. Referring to this Figure, shutter drive member 101 in the form of a disc is fixedly mounted on a single, rotatable shaft 102 on which a prime mover spring 103 is disposed. One end 103a of spring 103 is secured to the drive member 101 and its other end 103b is fixedly connected with a charging member 110 which rotates in association with a film winding operation. The spring 103 is charged in response to a film winding operation, whereby the drive member 101 is urged to rotate counter-clockwise, as indicated by an arrow. Before the shutter is operated, the resulting rotation of the drive member 101 is blocked by the abutment of an arm 101a extending from the drive member 101 against a detent member 105 which is rockably mounted on a stud 104. In this manner, the drive member 101 is maintained in its start position for performing a shutter operation.
Another detent member 106 has its own arm 106a located in the path of rotation of the arm 101a. The detent member 106 is pivotally mounted on a stud 107 and is urged by spring 108 to rotate counter-clockwise about stud 107. An armature piece 106b is formed on the end of the other arm and is held in abutment against an electromagnet 109.
In operation, when detent member 105 is moved out of abutting engagement with the arm 101a in response to shutter release, the drive member 101 rotates counter-clockwise under the resilience of spring 103 to open the shutter. In the fully open position of the shutter, arm 101a abuts against arm 106a of the other detent member 106, thus temporarily maintaining the shutter fully open. At this time, electromagnet 109 holds detent member 106 attracted thereto. When a proper exposure has been given, the electromagnet 109 is deenergized to permit the drive member 101 to rock the detent means 106 so as to move past it, thus undergoing a remaining one-half revolution about the shaft 102 until it returns to its start position, thus closing the shutter.
With the shutter mechanism of single shaft rotating type described above, the shutter drive member 101 is maintained at rest in its start position by engaging the arm 101a with detent member 105. The detent member 105 is in the form of a plate-like lever which is mounted to oscillate in the vertical plane and has a broad side against which one side of the arm 101a engages in face-to-face contact manner. Upon shutter release, this causes a twisting action to occur between the arm 101a and detent member 105, resulting in an unstable release operation. Specifically, since arm 101a is strongly urged to rotate counter-clockwise by the spring 103, it abuts against detent member 105 with a force of sufficient strength to present a large resistance to a sliding movement between the members 101 and 105, resulting in the twisting action when the release operation takes place.
To achieve a high speed shutter operation, it is desirable that drive member 101 be capable of rotating through one revolution at its maximum speed without experiencing any constraint or engagement. However, with the conventional mechanism illustrated in FIG. 1, it is once engaged by the detent member 106 adjacent to the fully open shutter position, presenting a difficulty to the achievement of a high speed shutter operation and causing a bouncing of shutter blades.
It will be noted that a camera incorporating an electrical shutter usually includes a variety of electrical accessory circuits including a fixed exposure period selection circuit which permits a synchro-flash photography, an exposure factor modification circuit which may be utilized during the photographing operation under the rear light illumination, a battery check circuit, an electrical self-timer circuit or the like, all of which are connected in circuit with the electrical shutter circuit. These accessories circuits are connected with the shutter circuit by using a changeover switch. By way of example, FIG. 2(A) shows a fixed exposure period selection circuit which may be connected with the shutter circuit. Specifically, the shutter circuit includes capacitor 201 which forms part of a photometric circuit and which is connected with the movable contact of changeover switch 202 having fixed contacts which are in turn connected with photometric, light receiving element 203 and with resistor 204 which is used to provide a fixed exposure period. During a flash photography operation, the switch 202 is thrown to the fixed contact associated with resistor 204 to form a time constant circuit with capacitor 201 and resistor 204, thus establishing a fixed exposure period for operating a comparator 205. FIG. 2(B) shows an automatic exposure control circuit 206 and a battery check circuit 207 which are selectively connected across a battery through changeover switch 208.
It will be appreciated that such additional means which are connected through changeover switches cannot operate properly if the switch fails. In addition, the failure of such switch may prevent a proper functioning of the inherent shutter circuit.